Plant architecture plays a very important role in overall crop performance. The characteristics of the inflorescence, flower, silique/fruit, and stem internodes have broad agronomic implications in the overall productivity of any crop plant. Compact architecture can contribute to productivity. For example, flowering stalks or inflorescences that are compact in nature and do not shade lower photosynthetic tissue can allow for greater productivity. Similarly, a flowering stalk or inflorescence that is spread out may allow for more photosynthesis to take place during seed development within the flowering stalk. Thus, different inflorescence architectures may be desired for different crops.
Since most crop varieties have been derived directly or indirectly through breeding from wild species, productivity of crops may be affected by characteristics that are evolutionarily beneficial to wild species but impair performance in an agricultural setting. For example, well spread-out flowers and siliques with long pedicels on the inflorescence (along with genes controlling seed dispersal mechanisms such as shattering) may be evolutionarily beneficial to wild species, while in a crop setting this confers significant disadvantages in terms of overall productivity as measured by harvested seed.
An example of this is canola species, in which the shoot architecture, especially involving inflorescence and siliques, is not ideal for optimal productivity and recovery of seed. Though there have been concerted efforts to produce crop plants with ideal architecture, it has not been achieved in many crop species.
It widely known that the growth and developmental programs of a plant species control pedicel development and determine its length, attachment angle of the flowers and seed pods, and contribute significantly towards the overall architecture of the flower and/or inflorescence. Despite significant advances in the understanding of flower development, very little is known about the genetic and molecular control of pedicel development.
Plant architecture or morphology is a major determining factor in plant productivity under agricultural settings. Plant varieties that have well-defined morphology of a uniform nature and pattern are preferred since they are amenable to mechanical cultivation. In particular, plant species that produce seed are selected for the uniformity of the placement of seed forming structures (typically seed pods or cobs) to allow efficient mechanical harvesting of seed. Plant varieties are also selected on the basis of other seed forming characteristics, such as strong pods to ensure no seed is lost or dispersed prior to harvesting, or compact nature of the raceme of the plant that contains the seedpods. Not all plants have these ideal characteristics. Thus, there is a strong interest in modifying the placement of seed pods and overall physical characteristics, of many seed plants to produce plants with desirable plant architecture and overall morphology. Compact plants, with clustered seed pods can provide many benefits for mechanical production of the crop, as well as lead to increased productivity. Accordingly, control of plant form and plant architecture is a desirable goal for the industry.
The building blocks of the plant architecture (body plan) are composed of reiterative units referred to as phytomers and these are elaborated during different phases of development (Sussex, I. M. & Kerk, N. M. (2001) Curr. Opin. Plant Biol. 4, 33-37). In Arabidopsis thaliana, three types of phytomers have been described (Schultz, E. A. & Hauglm, G. W. (1991) Plant Cell 3, 771-781.). The variations in the number of units and their size among these three main types of phytomers in different plant species contribute to the tremendous architectural diversity observed in flowering plants (Steeves, T. A. & Sussex, I. M. (1989) Patterns in plant development (Cambridge University Press, Cambridge). The activity of the shoot apical meristem (SAM), together with additional meristems, regulates the growth and development of all three types of phytomers (Medford, J. I., Behringer, F. J., Callos, J. D. & Feldmann, K. A. (1992) Plant Cell 4, 631-643 & Simon, R. (2001) Semin. Cell Dev. Biol. 12, 357-362). The SAM contains three major domains defined by cytoplasmic densities and cell division rates: the central zone (CZ), which is responsible for maintaining the pluripotent stem cells; the peripheral zone (PZ), which is involved in the production of lateral organs; and the rib zone (RZ), from which the bulk of the stem is derived (Bowman, J. L. & Eshed, Y. (2000) Trends Plant Sci. 5, 110-115). Recent studies in Arabidopsis have shown that several genes, including SHOOTMERISTEMLESS (STM), WUSCHEL and CLAVATA-family receptor kinases and their putative ligands define key functions in the SAM (Brand, U., Hobe, M. & Simon, R. (2001) BioEssays 23, 134-141., Long, J. A., Moan, E. I., Medford, J. I. & Barton, M. K. (1996) Nature 379,66-69, Mayer, K. F., Schoof, H., Haecker, A., Lenhard, A., Jurgens, G. & Laux, T. (1998) Cell 95, 805-815, & Clark, S. E. (2001) Nat. Mol. Cell Biol. 2, 276-284.)
In Arabidopsis the inflorescence constitutes the major part of the shoot and thus contributes significantly to the overall shoot architecture. Several genes have been identified in Arabidopsis that play key roles in defining the architecture of the shoot/inflorescence. For example, dwarf plants with uniform effects on all phytomers have been associated with altered levels of or defects in the signaling pathways of certain plant hormones (gibberellins or brassinosteriods—Hedden, N. P. & Kamiya, Y. (1997) Annu. Rev. Plant Physiol. Plant Mol. Biol. 48, 431-460, & Richards, D. E., King, K. E., Ait-ali, T. & Harberd, N. P. (2001) Annu. Rev. Plant Physiol. Plant Mol. Biol. 52, 67-88, and references therein). The supershoot (Tantikanjana, T., Yong, J. W., Letham, D. S., Griffith, M., Hussain, M., Ljung, K., Sandberg, G. & Sundaresan, V. (2001) Genes Dev 15, 1577-1588) and altered meristem program (Chaudhury, A. M., Letham, S., Craig, S. & Dennis; E. S. (1993) Plant J. 4, 907-916) mutants display abnormally high levels of cytokinins and produce extensive branching and altered shoot and inflorescence architecture. Auxin polar transport mutants, such as pinformed (Okada, K., Ueda, J., Komaki, M. K., Bell, C. J. & Shimura, Y. (1991) Plant Cell 3, 677-684) and pinoid (Bennett, S. R. M., Alvarez, J., Bossinger, G. & Smyth, D. R. (1995) Plant J. 8, 505-520), form inflorescences that are reduced to pin-like structures that do not produce any lateral organs or meristems. A compact inflorescence is caused by the erecta mutation, which involves a putative receptor kinase (Torii, K. U., Mitsukawa, N., Oosumi, T., Matsuura, Y., Yokoyama, R., Whittier, R. F. & Komeda, Y. (1996) Plant Cell 8, 735-746).
An even stronger effect on inflorescence architecture is conferred in a Landsberg erecta (Ler) background by the brevipedicellus (BP) mutation, which is defined by a recessive mutant with compact intemodes and short, downward-pointing pedicels (Koornneef, M., Eden, J. v., Hanhart, C. J., Stam, P., Braaksma, F. J. & Feenstra, W. J. (1983) J. Hered. 74, 265-272). Thus, mutants that exhibit altered architecture provide an indication that architecture can be altered, but there is no indication as to the molecular nature of the gene or the mechanisms by which these changes are manifested.
The role of homeobox genes in defining body plan and their evolutionary relationships in animals is well documented (Gehring, W. J., Affolter, M. & Burglin, T. (1994) Annu. Rev. Biochem. 63, 487-526, Kappen, C. (2000) Proc. Natl. Acad. Sci. USA 97, 4481-4486.) More recently, several plant knotted-like homeobox (KNOX) genes have been identified, which form two classes based upon sequence similarities and expression domains (Bharathan, G., Janssen, B., Kellogg, E. & Sinha, N. (1999) Mol. Biol. Evol. 16, 553-563, Reiser, L., Sanchez, B. P. & Hake, S. (2000) Plant Mol. Biol. 42, 151-166, Serikawa, K. A., Martinez-Laborda, A. & Zambryski, P. (1996) Plant Mol. Biol. 32, 673-693.)
In Arabidopsis, there are four different class I KNOX genes, STM, KNAT1, KNAT2, and KNAT6 (Long, ibid., Lincoln, C., Long, J., Yamaguchi, J., Serikawa, K. & Hake, S. (1994) Plant Cell 6, 1859-1876 & Semiarti, E., Ueno, Y., Tsukaya, H., Iwakawa, H., Machida, C. & Machida, Y. (2001) Development 128, 1771-1783.) STMis expressed in the SAM, whereas KNAT1 and KNAT2 expression observed in the PZ of the SAM. KNAT1 is also expressed in the cortical cell layers of the peduncle and pedicel. STM, KNAT1 and KNAT2 expression is excluded from the leaf primordia and developing leaves by ASYMMETRICLEAVES 1 and 2 genes (Ori, N., Eshed, Y., Chuck, G., Bowman, J. L. & Hake, S. (2000) Development 127, 5523-5532, & Byrne, M., Barley, R., Curtis, M., Arroyo, J., Dunham, M., Hudson, A. & Martienssen, R. (2000) Nature 408, 967-971). Ectopic expression of KNAT1 and KNAT2 in leaves induces altered symmetry and cell fate, and ectopic meristem/shoot formation from the adaxial surface (Chuck, G., Lincoln, C. & Hake, S. (1996) Plant Cell 8, 1277-1289). To date, loss-of-function mutations in class I KNOX genes are known only for STM and these suggest a critical role in SAM maintenance and function. Significantly, however, no such mutations have previously been described for KNAT1, hampering study of the role of this homeobox gene in plant development.
The future prospects of engineering optimal plant architectures in plant species will depend on the availability of critical morphology controlling genes and knowledge of their functional regulatory properties. For example in canola, the occurrence of an inflorescence and silique with long pedicels may offer some unique challenges and opportunities to develop an ideal architecture for improving productivity.
In summary, there remains a continuing need to develop novel and efficient techniques for modifying the morphology and architecture of plants, such as for example Brassica and other plant types, to improve photosynthetic efficiency, overall yield, and harvestability. This need extends to both crops and to horticulturally grown species to improve aesthetic appeal.